Breast cancer is the most commonly diagnosed cancer in US women. Notably, premenopausal African- American women have a significantly higher incidence rate of breast cancer compared to Caucasian- American, and a higher mortality rate at any age. Reports show young African-American patients have a higher incidence of the basal-like triple-negative breast cancer. Currently, no effective molecular therapies exist for this highly aggressive cancer and, consequently, patient survival is poor. The primary significance of the proposed study is its focus on identifying and testing novel translational targets for treatment of this aggressive, basal-like breast cancer. Previous studies show high CD44 expression is a critical component of stem-like, xenograft-initiating cells in basal-like breast cancer; therefore, defining the role of CD44 in these tumor-initiating cells is a essential step in developing targeted therapeutics. Preliminary results show an upregulation of CD44, and unique CD44 variant expression, in basal-like breast cancer compared to luminal breast cancer subtypes. Additionally, preliminary studies have identified a novel exotoxin, commonly associated with enteric diseases of mammals, that selectively binds high CD44 expressing breast cancer cells, alters CD44 signaling and induces cell death. Therefore, this proposal intends to functionally determine the mechanisms of this toxin's interaction with CD44 and to determine its potential as a therapy for high CD44 expressing, basal-like breast cancer. During the first phase of this proposal, the total levels and variant expression profile of CD44 wil be defined in luminal and basal-like breast cancer phenotypes using absolute quantitative RT-PCR, flow cytometry and immunohistochemistry in cell lines and primary breast cancer samples. This information will be used to define the toxin's target population of cells, and correlate the sensitivity of cancer cells to toxin exposure with CD44 expression and cancer phenotypes (Aim 1). The second phase of this study proposes the initial in vivo xenograft experiments necessary for translational development of the toxin for cancer therapeutics (Aim 2). The data generated from this proposal will provide the foundation of future proposals with the overall goal to develop novel targeted therapies for treatment of aggressive, basal-like breast cancer, particularly in young African-American women.